Learning Objectives
What is fertilization?
Fertilization is the process by which male and female gametes are fused together, initiating the development of a new organism.
The male gamete or 'sperm' and the female gamete 'egg' or 'ovum' are specialized sex cells, which fuse together to begin the formation of a zygote during a process called sexual reproduction.
The fertilization process in animals can occur either internally or externally, a difference that is largely determined by the method of birth. Humans provide an example of internal fertilization whereas seahorse reproduction is an example of external fertilization.
External fertilization
External fertilization usually occurs in aquatic environments where both eggs and sperm are released into the water. After the sperm reaches the egg, fertilization takes place.
Most external fertilization happens during the process of spawning where one or several females release their eggs and the male(s) release sperm in the same area, at the same time. The release of the reproductive material may be triggered by water temperature or the length of daylight. Nearly all fish spawn, as do crustaceans (such as crabs and shrimp), mollusks (such as oysters), squid, and echinoderms (such as sea urchins and sea cucumbers).
Pairs of fish that are not broadcast spawners may exhibit courtship behavior. This allows the female to select a particular male. The trigger for egg and sperm release (spawning) causes the egg and sperm to be placed in a small area, enhancing the possibility of fertilization.
External fertilization in an aquatic environment protects the eggs from drying out. Broadcast spawning can result in a greater mixture of the genes within a group, leading to higher genetic diversity and a greater chance of species survival in a hostile environment. For sessile aquatic organisms like sponges, broadcast spawning is the only mechanism for fertilization and colonization of new environments. The presence of the fertilized eggs and developing young in the water provides opportunities for predation resulting in a loss of offspring. Therefore, millions of eggs must be produced by individuals, and the offspring produced through this method must mature rapidly. The survival rate of eggs produced through broadcast spawning is low.
Internal Fertilization
Internal fertilization occurs most often in land-based animals, although some aquatic animals also use this method. There are three ways that offspring are produced following internal fertilization.
Internal fertilization has the advantage of protecting the fertilized egg from dehydration on land. The embryo is isolated within the female, which limits predation on the young. Internal fertilization enhances the fertilization of eggs by a specific male. Fewer offspring are produced through this method, but their survival rate is higher than that for external fertilization.
| External fertilization | Internal fertilization |
| The fusion of male gamete (sperm) and female gamete (ovum) occurs outside the body | The fusion of gametes occurs inside the body |
| Both individuals discharge their gametes outside the body | Only the male discharges sperms into the female genital tract |
| Development occurs outside the body | Development occurs inside the body |
| Chances of survival of the offspring are less. Therefore, a large number of eggs are produced | Chances of survival of the offspring are more. Therefore, a small number of eggs are produced |
| For example, frog, fish | For example, humans, birds, cow, hen |
Fertilization in plants occurs after pollination and germination. Pollination occurs through the transfer of pollen - which is the male microgametes of seed plants, producing the sperm - from one plant to the stigma (the female reproductive organ) of another. The pollen grain takes up water and germination occurs.
The germinated pollen grain sprouts a pollen tube, which grows and penetrates the ovule (the egg structure of the plant) through a pore called a micropyle. The sperm is then transferred through the pollen tube from the pollen.
In flowering plants, a secondary fertilization event takes place. Two sperm are transferred from each pollen grain, one of which fertilizes the egg cell to form a diploid zygote. The nucleus of the second sperm cell fuses with two haploid nuclei contained within a second female gamete called the central cell. This second fertilization forms a triploid cell, which subsequently swells and develops a fruiting body.
The process of fertilization, which involves the cross-fertilization between gametes from two different individuals, male and female, is called allogamy. Autogamy, also known as self-fertilization, occurs when two gametes from one individual fuse; this occurs in hermaphrodites, such as flatworms and certain plants.
Plant Fertilization
The fusion of two dissimilar sexual reproductive units (gametes) is called fertilization. This process was discovered by Strasburger (1884).
1. GERMINATION
Germination of pollen grain on stigma and growth of pollen tube: Pollen grains reach the receptive stigma of the carpel by the act of pollination. Pollen grains, after getting attached to the stigma, absorb water and swell. Subsequent to mutual recognition and acceptance of pollen grains, the pollen grain germinates (in vivo) to produce a pollen tube which grows into stigma towards the ovarian cavity.
G. B. Amici (1824) discovered the pollen tube in Portulaca oleracea. Generally, only one pollen tube is produced by a pollen grain (monosiphonous). But some plants like members of Cucurbitaceae produce many pollen tubes (polysiphonous). The pollen tube contains a vegetative nucleus or tube nucleus and two male gametes. Later, the vegetative cell degenerates. The pollen tube now reaches the ovule after passing through the style.
2. ENTRY OF POLLEN TUBE INTO OVULE
After reaching the ovary, the pollen tube enters the ovule. Pollen tube may enter the ovule by any one of the following routes:
a. Porogamy - When the pollen tube enters the ovule through micropyle, it is called porogamy. It is the most common type, e.g. Lily
b. Chalazogamy - The entry of the pollen tube into the ovule from the chalazal region is known as chalazogamy. Chalazogamy is less common, e.g. Casuarina, Juglans, Betula, etc.
c. Mesogamy - The pollen tube enters the ovule through its middle part i.e., through integument (e.g. Cucurbita, Populus) or through funicle (e.g. Pistacia).
3. ENTRY OF POLLEN TUBE INTO EMBRYO SAC
The pollen tube enters the embryo sac only from the micropylar end irrespective of its mode of entry into the ovule. The pollen tube either passes between a synergid and the egg cells or enters into one of the synergids through the filiform apparatus. The synergids direct the growth of the pollen tube by secreting some chemical substances (chemotropic secretion). The tip of the pollen tube enters into one synergid. The penetrated synergid starts degenerating. After penetration, the tip of the pollen tube enlarges and ruptures releasing most of its contents including the two male gametes and the vegetative nucleus into the synergid.
4. DOUBLE FERTILIZATION
The nuclei of both the male gametes are released in the embryo sac. One male gamete fuses with the egg to form the diploid zygote. The process is called syngamy or generative fertilization.
The diploid zygote finally develops into an embryo. The other male gamete fuses with the two polar nuclei (or secondary nucleus) to form the triploid primary endosperm nucleus. The process is called triple fusion or vegetative fertilization. These two acts of fertilization constitute the process of double fertilization. Double fertilization occurs in angiosperms only.
Fertilization Process in Animals
Fertilization is the process in which a single haploid sperm fuses with a single haploid egg to form a zygote. The sperm and egg cells each possess specific features that make this process possible.
The egg is the largest cell produced in most animal species. A human egg cell is approximately 16 times larger than a human sperm cell. The eggs of different species contain varying amounts of yolk, nutrients to support the growth of the developing embryo. The egg is surrounded by a jelly layer, composed of glycoproteins (proteins that have sugars stuck to them), that releases species-specific chemoattractants (chemical attractors) that guide sperm to the egg. In mammals, this layer is called the zona pellucida. In placental mammals, a layer of follicular cells surrounds the zona pellucida. The zona pellucida/jelly layer is separated from the egg by a membrane called the vitelline envelope, which is outside of the cell's plasma membrane. Just underneath the egg's plasma membrane are cortical granules, vesicles containing enzymes that will degrade the proteins that hold the vitelline envelope around the plasma membrane when fertilization occurs.
The sperm is one of the smallest cells produced in most animal species. The sperm consists of a head containing tightly packed DNA, a flagellar tail for swimming, and many mitochondria to provide power for sperm movement. The plasma membrane of the sperm contains proteins called bindin, which are species-specific proteins that recognize and bind to receptors on the egg plasma membrane. In addition to the nucleus, the sperm head also contains an organelle called the acrosome, which contains digestive enzymes that will degrade the jelly layer/zona pellucida to allow the sperm to reach the egg plasma membrane.
To ensure that the offspring has only one complete diploid set of chromosomes, only one sperm can fuse with one egg. Fusion of more than one sperm with an egg or polyspermy is genetically incompatible with life and results in zygote death. There are two mechanisms that prevent polyspermy: the "fast block" to polyspermy and the "slow block" polyspermy.
The above and the other steps of fertilization are discussed below:
